The invention relates to a tensioner for an endless power transmission member such as an endless belt, chain or the like and a power transmission system that includes such a tensioner and power transmission member such as a belt. More particularly, the invention relates to a spring type tensioner that biases the position of a pivot arm to which a pulley is rotatably mounted. While the tensioner of the invention may be used in various applications for tensioning an endless power transmission member, it is particularly useful as a compact tensioner in limited space applications for controlling tension of a toothed belt as associated with automotive camshaft drive systems.
In an automotive camshaft drive system, a toothed or synchronous belt is entrained around pulleys that include at least two toothed pulleys. One of the pulleys is a crank pulley and the other is one or more camshaft pulleys. An idler pulley such as a backside idler pulley may be pressed against the backside of the belt to effect an installed belt tension. Many automotive drive systems use a fixed idler pulley of the eccentric type where the eccentric pivot for the pulley lies radially inward of a bearing such as radially inward of the inner race of a ball type bearing. Since the pivot is radially inward of the bearing rotational surface, the bearings must be quite large to effect some amount of eccentricity for take up for tensioning a slacked belt. While a fixed eccentric type pulley does not introduce tension changes because of an angular positioning of a pivot arm because the eccentric is fixed, they do introduce another type of tension problem. The problem with a fixed eccentric type pulley is that there is a change in the belt drive pulley center distance between a cold engine such as usually occurs during engine start-up in a warm or thermally expanded engine such as occurs during normal engine operating temperatures. Thus, the thermal effect is to increase belt tension with engine temperature increases, and conversely, decrease belt tension with decreases in engine temperature. A self-adjusting belt tensioner can overcome such changes in the thermal affect between a hot engine and a cold engine.
An example of a self-adjusting timing belt tensioner is disclosed in U.S. Pat. No. 4,917,655. This tensioner being of the eccentric type with its pivot radially inward of the bearing surface, has the same disadvantage as a fixed eccentric type pulley and that is it requires a very large bearing. Typically, large bearings are usually more expensive than a smaller bearings. Also, in such self adjusting eccentric type tensioners, the length of the pivot arm effecting eccentricity is quite small. The short pivot arm length may limit the amount of available "take-up" and also introduce geometric force effects. Large angle changes of a short pivot arm must somehow be accounted for or balanced (e.g., with changing spring loads) for controlling belt tension.
U.S. Pat. Nos. 4,551,120 and 4,696,663 offer solutions to the short pivot arm of eccentric type tensioners by providing a longer pivot arm length. This is accomplished by locating the pivot radially outward of the pulley bearing surface. While such tensioners are compact, they are designed for front end accessory drive systems instead of timing belt systems and they have an axial length along the pivot axis which is quite deep. Such tensioners use a helically wound torsional springs to bias the positioning of the pivot arm. A housing for containing the torsional spring accounts for a large axial length. The axial length would make them unsuitable for some automotive belt tensioning applications because of insufficient space.
Another type of belt tensioner is disclosed in U.S. patent application Ser. Nos. 07/588,633 and 07/588,245. In these disclosures, the tensioners have a suitable axial length or depth for some automotive camshaft drive configurations, but their design introduces some geometric problems that may prevent them from being used in other applications. In both of the tensioners, the pulley is recessed in the tensioner structure to an extent that it is at least partially surrounded by projections of the tensioner's structure. Such projections must be avoided by a belt as it entrained around the tensioner pulley and other pulleys of the belt drives. The projections may substantially limit the applications for such tensioners because of space limitations of an automotive engine.